The purpose and meaning of the parable, "Parable of the Prodigal Son" is, Jesus is trying to teach others about God the Father's love for others. God the father is willing to still except everyone, even though everyone make mistakes.
The purpose and meaning of the parable "Muddy Road" is not to carry around mental baggage and just let go. The mind is the real battle ground, not nessasarily your actions.
The literary structure of Biblical Parables are very straight forward. They normally do not include similes or metaphors, but do include lots of dialogue. The literary structure pertains to one specific religion, Christianity.
The literary structure of Zen parables is open minded. You can interpret your own analysis without being apart of one specific religion. These usually include an inside meaning that teaches a life lesson.